The present invention relates to a method for measuring the convergence of a color cathode ray tube (CRT) and apparatus thereof, and more particularly, to a method for measuring the convergence, in which the influence of a relative position control with respect to the target CRT for measurement and the apparatus used in measuring the convergence can be minimized, and an apparatus for implementing the same.
In general, in a process for manufacturing a color CRT used as a display device such as a color television receiver or a color monitor, in order to reconstruct clearly the color of an image to be displayed, convergence is necessary so that the three primary color electron beams (R, G and B beams) emitted from a triple electron gun are adjusted to focus on a pixel point of a CRT screen at different incident angles.
The convergence is usually classified into static convergence which is adjusted for overall misconvergence (the center of an image) and dynamic convergence which is adjusted for partial misconvergence (the peripheral portion of an image).
Because of the need for convergence in the conventional color CRTs, to produce a good convergence state of the three electron beams on the screen, a strong pin cushion-like horizontal deflection magnetic field and a strong barrel-like vertical deflection magnetic field are created in the deflection yoke installed between the triple electron gun and the screen. The direction of the three beams is changed via these magnetic fields, to converge on the screen.
However, in practice, the electron beams emitted from the electron gun of a CRT are deflected by the deflection yoke installed in the neck portion of the CRT to produce raster on the screen. This causes a bad screen state such as misconvergence or generates electron beam landing due to the minuteness of assembly of the deflection yoke or CRT, instrument mechanical error or curvature difference of the CRT panels.
Japanese Patent laid-open publication No. Hei 3-217192 discloses a conventional convergence measuring apparatus for measuring and adjusting convergence, an extracted portion of which is shown in FIG. 1.
Referring to FIG. 1, the conventional convergence measuring apparatus includes a camera 12 for reading an image, a signal source 11 for generating cross-hatch pattern on a CRT 10, a digital filter 13 for separating the image read by camera 12 into red (R), green (G) and blue (B) beams, a set of memories 14 for storing each separated picture and a monitor 15 for display the stored picture in each memory 14.
The conventional convergence measuring apparatus having the aforementioned structure measures misconvergence such that cross-hatch pattern is generated on the screen of the target CRT for convergence measurement, the images of the local parts of an imaged region are read by the camera 12 and then the luminance centers of the R, G and B beams is extracted.
Also, as shown in FIG. 2, in order to measure the misconvergence of horizontal and vertical R, G and B beams, a horizontal window 10h and a vertical window 10v are set on the screen of the target CRT and the luminance of each internal R, G and B beams is calculated by a gravity center method to then be extracted.
The extent of measured convergence by the aforementioned measuring method and apparatus thereof is ordinarily within 20 .mu.m. Since the local parts of the screen of the CRT are imaged by a camera in order to increase the measuring preciseness, it is necessary to control the positions of the camera and the target CRT set for measurement of convergence.
However, when a color CRT is manufactured in practice, the process of measuring and adjusting the convergence thereof is generally performed in the state where a set of an assembly composed of a CRT and printed circuit board (to be called "CRT set" hereinafter) is put on a conveyer in its entirety by a predetermined device.
A position error of about 2 mm is practically generated either forwards or backwards, left or right and up or down, in controlling the position of the CRT set put on the conveyor as described above. Also, a position error of about 3 mm is generated during the adjustment depending on the horizontal and vertical position with respect to the curvature surface of the CRT screen and its size. In consideration of the partial imaged region of the CRT screen, which is about 10 mm, even such a minute positional error disables precise position control. Moreover, if the position control is incorrect due to an ambient factor such as a vibration or shock, the cross-hatched patterns of a predetermined region in the screen of the CRT are then outside the imaged region of the camera, thereby preventing the camera from imaging the cross-hatched patterns precisely.
Thus, whenever the operator intends to measure and adjust the convergence of a new CRT set, the convergence should be adjusted by manually controlling the relative position with respect to the CRT set and the camera used for measurement. As a result, operating efficiency is lowered, which ultimately decreases productivity.